In recent years, in the field of mobile communication, a high speed down link packet access (HSDPA) of W-CDMA, an OFDM (Orthogonal Frequency Division Multiplex) modulation scheme for improving a frequency use efficiency and the like have been used to improve transmission efficiency. The HSDPA, the OFDM modulation method and the like use the M-ary QAM (Quadrature Amplitude Modulation) scheme. Hence, it is demanded that this scheme have a signal quality (modulation accuracy: EVM or the like) that meets stricter standards than those which a conventional QPSK (Quadrature Phase Shift Keying) scheme is required to meet.
Generally, a low back-off operation of a power amplifier needs to be realized for a purpose of minimizing a transmission device and reducing power consumption. Hence, the power amplifier adopts a peak suppressing circuit which reduces a peak factor of a transmission signal. The peak suppressing circuit tolerates slight quality deterioration of a transmission signal, and performs signal processing for suppressing peak components.
A peak suppressing circuit disclosed in Patent Literature 1 will be described below. The peak suppressing circuit receives an IQ signal. The IQ signal is a signal having in-phase components and quadrature components, and is a signal whose spectrum band is limited to satisfy each carrier signal requirement per transmission system. The peak suppressing circuit detects an amplitude value of the received IQ signal, and determines a peak position which needs to be suppressed. Further, the peak suppressing circuit subtracts a generated signal (referred to as a peak suppression signal below) from the inputted IQ signal to suppress a peak. The band of the peak suppression signal is limited by using a FIR filter to make a signal of a frequency band which is substantially the same as a spectrum band of the IQ signal or settles in a range of the spectrum band.
In this regard, when the FIR filter which limits a band receives an input of a plurality of continuous peak pulses, an output response of a band limiting filter to a peak pulse input at a given point among these continuous peak pulses is larger than the amplitude of an original peak pulse due to a convolution influence (an overlap of impulse responses).
Hence, the peak suppressing circuit multiplies a peak pulse by a weight coefficient of a fixed value according to a peak pulse detection position, and reduces the amplitude of the peak pulse. Thus, the convolution influence in the band limiting filter is reduced.